Digital Subscriber line (DSL) systems are widely used to transmit digital data to users at home. Various standards such as ADSL and VDSL have been provided and are currently developed for transmitting the data in duplex communication such as for example by using a discrete multitone modulation. Typically, a DSLAM at the operator's side (Digital Subscriber Line Access Multiplexer) provides connection over a plurality of lines to a plurality of transceivers at the costumer premise equipment (CPE). A DSLAM may be implemented in an Optical Network Unit (ONU),a Central Office (CO), a cabinet, an exchange or other types of network termination devices at the operator's end.
The line connecting the DSLAM and the CPE is typically a pair of twisted wires made of copper also sometimes referred to as twisted pair line. In order to enable communication over the line, loop maintenance is necessary and required by some regulatory bodies. Loop maintenance includes several measurements and tests, which are performed in order to determine the electrical line conditions and to discover any malfunction of the line. Such testing includes for example testing of electrical line parameters which are herein referred to as line tests. Furthermore, in order to prevent galvanic corrosion of the metallic lines and oxidization at splices, a small DC line current also referred to as wetting or sealing current may be provided periodically.
In existing systems, DSL is provided together with plain old telephone service (POTS) by utilizing different frequency bands for POTS and DSL and separating the signals by means of filters. While the DSL system is connected to the line via a high-pass filter and a transformer, POTS still provides a DC-connection, which can be used to perform line tests or to directly inject the wetting current into the line. However, with the increasing trend of utilizing DSL based voice communication, such as Voice over IP (VoIP), future DSL systems will more and more be provided without POTS.